Yawning is a complex, automatic,
physiological phenomenon whereby the lungs are
fully expanded, the heart is to greater
activity, and, probably, the blood is charged
more fully in oxygen. It commences with an
involontary spasm of certain muscles of
mastication and deglutition; its termination is
akin to the process of sighing, and, like the
manifestation of deep breathing, it reinforces
respiration.

At night, when respiration is slckening, or
in the morning, when it has not recovered its
waking rhythm, yawning is accompanied sometimes
by the "stretching" of successive groups of
muscles the blood vessels of which have probably
been compressed by the previous assumption of a
constrained position; the local circulation is
thus assisted and stimulated.

The preacher, the novelist,, and the artist
emply the mechanism of yawning as an indication
of ennui, of lack of interest, or of wearied
attention, whether real or feigned. To the
clinician it should be signifiant od an attack
of asystole and consequent temporary anaemia of
the brain, especially of the corpus stritum. In
diseases where "air hunger" is a frequent symtom
yawning is also exhibited.

It is met with diabetes, in fatty and
fibroid changes in the walls of the heart, in
pericarditis, and in the pathological conditions
which lead to the faulty filling of the aorta.
Yawning suggests the proximity of a fatal ending
after copious heamorrhage and in pernicious
anaemia and Addison's disease. Dr Hughlings
Jackson recorded a remarkable demonstration
shortly after the introduction of the use of the
ophtalmoscope. He was surveying the fundus of an
eye when suddendly the field became pale. This
pallor was due obviously to a contraction of the
retinal blood vessels. He thereupon stood back,
expecting something to happen, and in fact the
patient immediately yawned. The observer had
seen, in part, a spasm of the cerebral
arteries.